Method and device for using extended interference fit screw shanks for spinal stabilization

ABSTRACT

A device and a method for stabilizing vertebrae in a human spine for the purpose of fixing one vertebra with respect to other vertebrae and with respect to other parts of the spinal column. This device comprises bone screws that clamp to a plate to maintain the plate in contact with the vertebrae. The device may be fabricated from non-metals, metal, alloys, or composite materials. A tapered screw head is pulled into the plate with the taper extending through the plate into the underlying bone. Extending the taper into the bone moves the screw thread stress raiser into an area of lesser deflection. This strengthens the fixation by increasing the rigidity and reducing the risk of screw breakage. Extending the taper into the underlying bone has also been shown to strengthen the taper lock.

CROSS-REFERENCES TO RELATED APPLICATIONS:

[0001] This patent claims the benefit of U.S. provisional applicationSer. No. 60/271,782 filed Feb. 27, 2001.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not Applicable

REFERENCE TO A MICROFICHE APPENDIX

[0003] Not Applicable

BACKGROUND OF THE INVENTION

[0004] This invention relates generally to implantable medical devicesand their methods of use for stabilizing skeletal bone, and relates moreparticularly to fasteners for attaching implantable medical devices andtheir use for stabilizing the vertebrae of a human spine.

[0005] With normal anatomy, the vertebrae of the spinal column are heldtogether and to the skeleton by a complex arrangement of ligaments,tendons and muscles. Degenerative diseases, deformities, or trauma maycause abnormal conditions. These problems generally cause or allowdisplacement or rotation of a vertebra relative to the adjacentvertebrae, or produce painful motion. When spinal discs tear, rupture orbulge the intervertebral space between two adjacent vertebras candecrease or displace abnormally and cause discomfort to the patient.When surgery is needed, the discs are replaced with implants that willheal or “fuse” the spine together. This device, with its associatedstabilization, maintains the vertebral position while healing takesplace. The result is referred to as “spinal fusion”. The objective ofspinal implants is to facilitate or maintain realignment and/or fixationof spinal elements. Clinical studies have demonstrated that surgeriesusing spinal implants are more effective at maintaining alignment andproviding rigidity to the spine than surgeries in which implants are notused. Since the introduction of stabilizers as crude plates, rods, andwires, these devices have been developed into sophisticated appliances,which can be assembled and configured to rigidize spines of any size orcondition. These stabilizers also provide mechanical fixation forrestraint of an implanted graft material. With this fixation,displacement during healing is significantly reduced thereby reducingthe failure rate. Failure of the stabilizer commonly results from screwbreakage.

[0006] The majority of existing cervical stabilizers use plates that arebent in both the radial plain to conform to the vertebrae, and along thespinal axes to maintain lordosis. Bicortical screw purchase has beenfavored because of the increased strength of the construct and increasedscrew thread area within the bone. These screws are more technicallychallenging to place and add increased risk of morbidity from neuralcanal penetration and screw backout. The reduced bending reactionstrength and decreased thread area of a unicortical screw purchaseincreases the probability of screw back out or loosening resulting inesophageal injury. Unicortical purchase results in a single point fixedend cantilever construct versus bicortical purchase which results in adual point fixed end construct. Screw back out and loosening has led tothe development of mechanisms for locking the screw head to the plate inunicortical screw plate designs. Such locking mechanisms not onlyprevent screw back out, they also reduce the tendency of the screw headto pivot within the plate. Locking the screw to a plate, rod or otherinsert results in a fixed point of attachment.

[0007] A second point of fixation results from the screw portion fixedwithin the bone. This produces a stress raiser of greatest stress, justbelow the screw attachment at the plate, rod or other implant. This isthe area of most frequent screw breakage observed in clinical practiceand biomechanical testing. This area of screw breakage is well known tothose practiced in the art. In the lumbar spine, posterior screws breakbetween the second and third thread below the plate or rod. In thecervical spine the anterior screws break at a similar location. In thepresent invention, particular embodiments are described below, Theseembodiments improve screw strength and lock the screw within the bone byextending a tapered unthreaded section of the screw shank into the bone.

SUMMARY OF THE INVENTION

[0008] A device and a method for stabilizing spinal vertebrae in a humanspine for the purpose of temporarily fixing the vertebra with respect toother vertebrae, a graft, and with respect to other parts of the spinalcolumn. The plate most commonly has a plurality of holes. The bone screwhas a threaded portion that engages a predrilled or threaded hole in thevertebra or the graft. The bone screw also has a non-threaded portionthat extends through the fixation plate, into the vertebrae, when thescrew is seated. The bone screw maintains the plate in contact with thevertebra. The bone screw non-threaded portion is pulled into a lockingmechanism on the plate. In the preferred embodiment the screw'snon-threaded portion consists of a taper which locks into a stabilizingplate. The locking mechanism of the plate consists of a matching taperinto which the screw shank is seated, locking the screw to the plate byan interference fit. The tapered or interference fit portion of thescrew extends through the plate into the vertebral bone and is describedas an extended shank.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The present invention will be understood better from thefollowing detailed description of the preferred embodiment. In theaccompanying drawings the reference numbers refer to the individualparts described in the text.

[0010]FIG. 1 is a side sectional view, at 1-1 of FIG. 3, of the spinalstabilization system shown implanted on the cervical portion of a humanspinal column with the screws placed at an angle to the spinal column.

[0011]FIG. 2 is a side sectional view, at 2-2 of FIG. 3, of the spinalstabilization system shown implanted on the cervical portion of a humanspinal column with the screws at 90 degrees to the spinal column.

[0012]FIG. 3 is a front (proximal) view of the spinal stabilizationsystem shown implanted on the cervical portion of a human spinal column

[0013]FIG. 4 is a front (proximal) view of the spinal stabilizationplate.

[0014]FIG. 5 is an end sectional view, at 5-5 of FIG. 3, of the spinalstabilization system shown with a screw in the left side with thevertebrae removed. The drill/tap bushing is shown in the right sidehole.

[0015]FIG. 6 is a side view of the spinal stabilization system screw.

[0016]FIG. 7 is an enlarged sectional view, of the encircled area 7 ofFIG. 2, with cylindrical bone and plate holes.

[0017]FIG. 8 is an enlarged sectional view of the encircled area 8 ofFIG. 2. With the plate hole cylindrical and the bone hole tapered.

[0018]FIG. 9 is an enlarged sectional view of the encircled area 9 ofFIG. 2. with the bone holes and plate holes tapered.

[0019]FIG. 10 is an enlarged sectional view of the encircled area 10 ofFIG. 2. with bone holes cylindrical and plate holes tapered.

DETAILED DESCRTION OF THE INVENTION

[0020] For simplification the stabilizer system is described as a singlelevel cervical stabilizer in one of many conceivable embodiments. Thatis not to imply that this is the only embodiment within which thestabilizing system can be configured. For consistency in this patent theword stabilizer or implant refers to the plate screw assembly or partsthereof, whereas the word graft refers to the material replacing theremoved disc or vertebrae. This device comprises a plate and bone screwsfabricated from metal, alloy, polymeric, plastic, biodegradable,bio-absorbable, human tissue, allograft, and autograft or compositematerial. The plate may be fabricated into a multilevel configuration.

[0021] The Device

[0022] Referring to FIGS. 1, 2, and 3 in the preferred embodiment, thesystem is attached to the anterior surface of the spine 29. The system10 may be modified for use on the lateral or posterior aspects of thespine. The system comprises plate 12 and bone screws 20. The system 10and its components are described in detail in the following paragraphs.The bone stabilizing method of implanting is described in a subsequentsection of this patent.

[0023] Referring to FIGS. 1, 2, and 3 in particular, the anteriorcervical plate system 10 is shown in combination with bone screws 20.Each of the plate 12 tapered holes 13 receive a bone screw. Bone screws20 each include a head 23, a threaded portion of the shank 21 and atapered shank portion 25, between the head 23 and the threads. Thetapered section 25 has a minor diameter that equals or exceeds the majordiameter of the threads of shank 21. These diameters allow the bonescrews 20 to be inserted, shank first, into any of the screw holes 13from the anterior side 11 of plate 12, with the threaded shank 21passing through the hole 13 of the posterior surface. The thread engagesa predrilled and pre-threaded hole 33 in the vertebra or the graft 30.The bone screw maintains the plate 12 in contact with the vertebrae 31and 32. The screw's tapered portion 25 is pulled into the matchingtapered plate hole 13 locking the screw 20 to the plate 12. The taper isconfigured to be self-locking preventing the screw from backing out. Thetaper is of sufficient length to extend into the vertebral bone. Thetaper will compress the bone hole resulting in increased strength of thebone-screw interface.

[0024] The Plate

[0025] The plate 12 is the framework upon which the bone screws 20 areattached. The plate 12 has two holes per vertebra, approximatelyperpendicular to the patient's spinal axis to receive and contain thebone screws 20. These screw holes may be angled to increase the preloadas shown in FIG. 1 or installed with no angle as shown in FIG. 2. Theplate has one hole for each screw tapered 13 or cylindrical 14. In thepreferred embodiment the plate 12 is fabricated from a single piece ofmaterial. In prior art these plates contained threads for locking thescrew or small locking devices such as cams were used to prevent thescrews from backing out under sustained movement of the patient. Toeliminate the use of plate threads on these materials the screw 20 isheld in place with a taper 13 or a interference cylindrical fit 14 inthe plate. The taper allows the use of the full thickness of the platefor a holding area. The plate may also be made in two or more levels.

[0026] The Bone Screw

[0027] In the preferred embodiment the bone screw, may use cylindricalor tapered bone screw threads 21, in the bone 31 and 32. As shown inFIGS. 7, 8, 9, and 10, the bone screw also has a section 22, tapered 17,or cylindrical 18, at the unthreaded portion of the shank, which engagesthe bone holes 15 or 16. The screw also has a tapered section 25 or acylindrical section 26 which engages the plate holes 13 or 14. The screwhas a feature 24 that will accept a driving tool. The driving featuremay be incorporated into the section within the portion of the screw,which engages the plate so as not to protrude into the esophagus. A bonescrew 20 is threaded into a drilled and tapped hole in a selectedvertebra 31 and 32 to fix it into position. The tapered shank portion 22extends into the vertebral bone, compressing the bone hole 15 or 16. Thescrew threaded distal portion passes through the plate. The screw may beself-tapping obviating the need for screw hole tapping. An alternatebone screw may have cylindrical shanks 18 or 26 with interference fitswithin the bone hole or the plate hole.

[0028] The Graft

[0029] After removing the disc and the cartilage, a graft 30, shown inFIGS. 1 and 2, preferably a non-degrading bone growth-compatiblematerial is positioned between the two vertebrae 31 and 32 in theintervertebral space. Such grafts are structurally load-bearing devices,capable of withstanding the compressive forces supported by the adjacentsuperior vertebra 31, however they will not withstand the tensile forceexperienced at the vertebral to graft interface. The stabilizer 10 andthe surrounding ligaments, tendons, and muscles must be preloaded tomaintain compression between the graft 30 and the adjacent vertebraduring any upper body motion that tends to put the vertebrae in tension.The graft 30 must be in compressive contact with the vertebral endplates 31 and 32. The graft 30 also may be metal, nonmetal, polymeric,allograft or autograft materials. A screw may be placed into the graftas well, with the tapered section extending into the graft material.

[0030] The Method

[0031] After the disc is removed the graft 30, as shown in FIGS. 1 and2, is forced into position at the center of the vertebral end plates 31and 32. Replacing damaged discs with rigid grafts is well known to thosepracticed in the art. The method of stabilizing the graft andmaintaining the relationship between the two vertebrae is still achanging technology. The plate is selected and placed on the patient'svertebra 31 and 32. A portion of the vertebral protrusion 28 may beremoved for a proper fit. The remainder of the method is presented asthree procedures listed below.

[0032] The first procedure (1) utilizes plates 12 with pre-formed holes13 or 14. In the second method (2) the plate contains no pre-formedholes. The plate holes are drilled or reamed into the plate at the timeof placement in surgery. The third procedure (3) utilizes a template toguide the drilling and tapping operations. On frequently used platesizes these templates may be used to align and position each drill andtap hole with respect to the other holes on the same plate. In allembodiments, the tapered thread shank screws may be self-tapping.Self-tapping screws are commonly used and are well known to thosepracticed in the art. Guide bushings and templates are used to align thedrills and threading taps and to serve to protect the plate holes duringdrilling and tapping of the bone.

[0033] Procedure #1

[0034] Bushings 41are inserted into the pre-formed plate holes 13 or 14to align the drill and thread tap and to protect the plate tapered hole.After inserting the bushings in the plate, the posterior side of theplate may be placed temporarily on the vertebra near the area where itwill be attached and repositioned to determine the best location for thescrews. The plate 12, with guide bushing 41 in place, as shown in FIG.5, is used as a template to guide the drill and tap at the position andangle of the matching plate holes. Once the holes are threaded 33, andthe guide bushings are removed, the screws 20 are partially threadedinto these bone holes 15 or 16. The screws are then tightened to engagethe plate locking mechanism and compress the extended tapered shank intothe vertebral bone.

[0035] Procedure #2:

[0036] The tapered plate hole 13 or the cylindrical hole 14 and the boneholes 15 or 16, shown in FIGS. 7, 8, 9, and 10, may be drilled at thetime of plate emplacement. In this procedure the plate 12 is positionedand then held using a positioning and/or holding means. This holdingmeans fixes the plate in the preferred position for subsequent drillingand tapping operations. Once the plate 12 is positioned, the plate andthe bone are drilled preferred position and direction through the plateand into the vertebrae 31 and 32 sequentially. A straight drill with atapered section may be incorporated into one tool bit to facilitatetaper placement. The holes are drilled through the plate and into thebone. In the preferred embodiment the taper is extended into the bone.The screw is then placed and tightened to engage the plate lockingmechanism and to engage the extended tapered shank into the vertebralbone.

[0037] Procedure #3.

[0038] In this procedure cylindrical bone hole 16 or the tapered bonehole 15 and thread hole 33, shown in FIGS. 7, 8, 9, and 10, are drilledthrough a template that closely approximates the plate contour and holeplacement. The drilling and threading operations are performed throughthe template thereby aligning with and protecting the existing holes 13or 14 in the plate 12. When the drilling and threading operations arecompleted and the plate is permanently positioned and secured with thescrews 20.

We claim:
 1. A cervical vertebral bone stabilizing system and method,for the purpose of fixing one vertebra with respect to one or more othervertebra or with respect to another implant or graft material within thehuman spine containing; (a) a fixation screw, pin, or other bonegrabbing device with a locking taper extending into the underlying bonefor a distance lmm or greater, and (b) two or more bone screwsconfigured to engage a rigidizing plate, and to engage previouslymachined holes within said vertebra, for the purpose of retaining saidplate to said vertebrae, and (c) said screws with a shank portion and ahead portion configured and sized to be affixed by an interference fitwithin said plate hole and also affixed within a portion of saidmachined holes into the vertebral bone, and (d) said screws extendingthrough the said plate from the anterior surface through the saidposterior surface holes engaging said machined bone screw holes forfastening the plate to at least two vertebral bodies of a human cervicalspine along the anterior side of the spine.
 2. The cervical stabilizingsystem of claim 1, wherein the said bone screws have a threaded portionto engage threads in the machined holes within the bone.
 3. The cervicalstabilizing system of claim 1, wherein the said interference fit of saidbone screws contain one or more locking tapers which extend into theunderlying bone to prevent screw backout.
 4. The cervical stabilizingsystem of claim 1, wherein the plate holes and the said vertebral holesare tapered to engage mating tapers of the bone screw of claim
 3. 5. Thecervical stabilizing system of claim 1, wherein the interference fitportion of the said screw shank or head extends 1 mm or more into thesaid plate hole.
 6. The cervical stabilizing system of claim 1, whereinsaid screws are fixed to said plate and vertebral bone holes withadhesive, cement, or other bonding materials.
 7. The cervicalstabilizing system of claim 1, wherein a hole is produced in saidvertebra by a drilling tool.
 8. The said hole of claim 1, whereas saiddrilling tool is aligned and guided by a guide bushing within the saidplate hole.
 9. The said hole of claim 1, whereas a thread is producedwith a thread forming tool.
 10. The said thread forming tool of claim 9,whereas the tool is aligned and guided by a bushing within said plate.11. The cervical stabilizing system of claim 1, wherein the said screwhead has driving feature to allow for tightening.
 12. The cervicalstabilizing system of claim 1, wherein the said screw has self tappingscrew threads.
 13. The cervical stabilizing system of claim 1, whereinthe screws and or stabilizing system are composed of metal, alloy,polymeric, plastic, biodegradable, bio-absorbable, human tissue,allograft, and autograft, composite, or combination of materials.
 14. Abone stabilizing system and method, for the purpose of fixing one bonewith respect to one or more other bones or with respect to anotherimplant or graft material within the human skeleton containing; (a) afixation screw, pin, or other bone grabbing device with a locking taperextending into the underlying bone for a distance 1 mm or greater, and(b) two or more bone screws configured to engage a fixating plate ordevice, and to engage previously machined holes within said bone, forthe purpose of retaining fixation to said bones, and (c) said screwswith a shank portion and a head portion configured and sized to beaffixed by an interference fit within said plate hole and also affixedwithin a portion of said machined holes in the bone, and (d) said screwsextending through a fixation device from the exterior surface throughthe holes engaging said bone screw holes for fastening the fixationdevice to bone.
 15. The bone stabilizing system of claim 14, wherein thesaid bone screws have a threaded portion to engage threads in themachined holes within the bone.
 16. The stabilizing system of claim 14,wherein said interference fit of said bone screws contain one or morelocking tapers which extend into the underlying bone to prevent screwbackout.
 17. The fixating system of claim 14, wherein the stabilizerplate holes and the said bone holes are tapered to engage mating tapersof the screw of claim
 15. 18. The stabilizing system of claim 14,wherein the interference fit portion of the said screw shank or headextends 1 mm or more into the said fixator.
 19. The stabilizing systemof claim 14, wherein said screws are affixed to said plate, or otherstabilizer, and to the bone holes with adhesive, cement, or otherbonding materials.
 20. The stabilizing system of claim 14, wherein ahole is produced in said bone by a drilling tool.
 21. The said hole ofclaim 20, whereas said drilling tool is aligned and guided by a guidebushing within the said stabilizer hole.
 22. The said hole of claim 20,whereas a thread is produced with a thread forming tool.
 23. The saidthread forming tool of claim 22, whereas the tool is aligned and guidedby a bushing within said plate or stabilizer.
 24. The bone stabilizingsystem of claim 14, wherein the said screw head has driving feature toallow for tightening.
 25. The bone stabilizing system of claim 14,wherein the said screw has self tapping threads.
 26. The bonestabilizing system of claim 14, wherein the screws and or stabilizingsystem are composed of metal, alloy, polymeric, plastic, biodegradable,bio-absorbable, human tissue, allograft, and autograft, composite orcombination of materials.